The most economical way yet get good sound is to spend 120pounds on the Eltax Nova all-in-one system at RS, which all owners are raving about, and it has RGB out as well, which Sony TV uses. As your outlay is very low, and hence your depreciation is lower as well. Very economical.
On the other hand if you get the Home Cinema bug you may quickly feel the need to upgrade.
Try getting a demo from a shop or friend to decide what sort of system would suit you best. Are you a fanatic who would spend hundreds just on mains cables or - no insult intended - one of those who would not recognise the difference between a £10 speaker and a £1000 one!
If you can categorise yourself you will then be able to assess how much it's worth spending for your own requirements. This could be a cheapish all-in-one or very expensive separates.
I presume from the 'economic' in the question that the budget is not limitless, so other factors like size of the system and whether anyone else has a say in the look/size come into play.
Essentially I'm on my 4th setup so I'd guess that not many get it right first time - but thats half the fun.
thanks for the info, the Eltax Nova all-in-one system seems great except for the dvd player, as ive already got one, is it worth getting another or can i get everything that comes with it except the dvd player for less anywhere?